Improving the photovoltaic performance of CdSe0.2S0.8 alloyed quantum dot sensitized solar cells using CdMnSe outer quantum dot

Improving the photovoltaic performance of CdSe0.2S0.8 alloyed quantum dot sensitized solar cells using CdMnSe outer quantum dot

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Improving the photovoltaic performance of CdSe0.2S0.8 alloyed quantum dot sensitized solar cells using CdMnSe outer quantum dot Maryam Ostadebrahim, Hossein Dehghani

Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, P.O. Box 87317-51167, Kashan, Iran



Keywords: Quantum dot sensitized solar cells Ternary CdSe0.2S0.8 QDs CdMnSe outer QD Cascade band structure

In this paper, by introducing the Mn-doped CdSe (CdMnSe) layer as outer quantum dot (QD) on ternary CdSe0.2S0.8 QDs surface, we developed an effective way to enhance the power conversion efficiency (PCE) of the CdSexS1-x alloyed quantum dot sensitized solar cells (QDSSCs) when the molar ratio of Se/Na2S·9H2O is 1:4. As a result, a cascade band structure and the midgap states which favorable for electron injection and the hole transport, are obtained when the concentration of Cd2+, Se2+ and Mn2+ ions are 0.5, 0.5 and 0.05 M, respectively, in the CdMnSe outer QD deposited by the successive ionic layer absorption and reaction (SILAR) method with three cycles. Hence, with using polysulfide electrolyte and Cu2S-brass as counter electrode, the measured PCE for the CdSe0.2S0.8/10%CdMnSe QDSSC is 5.420% (Voc = 0.70 V, Jsc = 16.834−2, and FF = 0.460) at AM 1.5G, which is higher than the PCE of 4.327% for the device with bare CdSe0.2S0.8 QDs or a ~25.5% increase. Our findings indicate that such improvement in PCE is caused by the increasing of lightabsorption, decrease of the surface roughness, improvement of electrons transfer from QDs to TiO2 CB, reduction of electrons recombination and thereby, the increasing collection of electrons in TiO2 film.

1. Introduction In order to solve the danger of global warming, emission of greenhouse gases and environmental pollution, the development of renewable energy sources must be taken into account (Choi et al., 2013; Dong et al., 2012; Duan et al., 2015; Holdren, 2008). Hence, the human society has been encouraged to use renewable clean energy sources, such as power from water, wind and solar energy. Among clean energy sources, the rational utilization of solar energy to provide the electric energy is of great importance for human in the future. A photovoltaic (PV) device is a solar cell, which on the basis of photoelectric effect, converts sunlight directly into electricity. Photovoltaic devices are generally classified into first-, second- and third-generation devices, and QD-based solar cells as third-generation PVs have attracted special attention because of their high absorption coefficient, multiple exciton generation (MEG) effect (Kim et al., 2013; Lior, 2008; Sambur et al., 2010; Tian et al., 2013), tunable band gap through size or composition control (Alivisatos, 1996; Samadpour et al., 2012; Zhu et al., 2013; Zhou et al., 2016), simple fabrication (Mora-Sero et al., 2009; Romeo et al., 2010), hot electron transfer, low cost (Kamat, 2008; Kamat, 2012; Kamat, 2013; Lee et al., 2013; Robel et al., 2006), and their higher light, moisture and thermal stability in comparison with lead halide peroskites and dye molecules (Pan et al., 2018). However, despite of all ⁎

these advantages, the power conversion efficiencies (PCEs) of QD-based solar cells are still in a low level (Kamat, 2008; O’regan and Grätzel, 1991; Rühle et al., 2010); the main reason for the little efficiency of QDSSCs is its relatively fast charge recombination process at the TiO2/ QDs/electrolyte interface (Bisquert et al., 2004; Tvrdy et al., 2011). Thus, many efforts have been made to enhance optical absorption (Cheng et al., 2012) and decrease the electron-hole recombination rate in QD-based solar cells (Roelofs et al., 2013). For instance, introducing an interlayer between the metal oxide electrode, typically TiO2 or ZnO, and QDs or deposition of the outer layer QDs with narrow band gap on the metal oxide/QDs electrode surface by the successive ionic layer absorption and reaction (SILAR) (Yang et al., 2012) method or chemical bath deposition (CBD) (Huang et al., 2011) method has been developed in recent works. For example, semiconducting CdS was applied as an interlayer for TiO2/CdS/CdSe (Şişman et al., 2017) and ZnO/CdS/CdSe (Tian et al., 2012) photoanodes. Also, the PbS QDs in the TiO2/PbS/CdS photoanode were used as an interlayer. Based on experimental results, presence of an interlayer with matched lattice parameters between the semiconductor electrode and QDs with thickness optimized could increase the sunlight absorption and decrease the recombination of electrons and holes, and hence lead to the rise of PCE in the QDSSCs because of the increased short-circuit current density (Jsc) and the electrons recombination resistance (Rrec) values, as well. As a result, the

Corresponding author. E-mail address: [email protected] (H. Dehghani). Received 25 April 2019; Received in revised form 20 June 2019; Accepted 16 October 2019 0038-092X/ © 2019 Published by Elsevier Ltd on behalf of International Solar Energy Society.

Please cite this article as: Maryam Ostadebrahim and Hossein Dehghani, Solar Energy,

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transmittance in the visible region, 3 mm thickness) glass substrates, Brass foil and scotch spacer (50 µm thickness) were purchased from Solaronix Company. Deionized (DI) water was generally used throughout the research.

stepwise structure in QDSSCs due to the presence of several different types of QDs could effectively improve the PCE. For example, Shu et al. used Pt counter electrode for CdSexS(1-x) and CdSexS(1-x)/CdSe solar cells and obtained energy conversion efficiencies (η) of 2.27% and 3.17%, respectively, an improvement of 40% (Shu et al., 2012). Chen et al. reported that a CdSxSe(1-x)/CdSe-sensitized TiO2 solar cell achieved a Jsc up to 14 mA·cm−2 and a η of 4.46%, an improvement of 48% in η compared to the device with bare CdSe QDs (3.00%) (Chen et al., 2014). Maity et al. reported that a CdS0.7Se0.3/CdS-sensitized solar cell (with Cu2S deposited ITO glass plates as CE) achieved a Jsc up to 15.49 mA·cm−2 and a η of 4.50%, an improvement of ~14% in η compared to the device based on the CdS0.7Se0.3 QDs (3.95%) (Maity et al., 2016). Yang and Zhong investigated the deposition of the CdSeS and CdS coating layers onto the CdTe QDs (via the SILAR method) in a core/shell structure of photoanode for solar cells with Cu2S-brass CE, which an increase of 22% and 35% in η was obtained respectively, compared to bare CdTe based QDSSC (Zhou et al., 2016). Zhou et al. reported introduction of CdSxSe1-x QDs between CdS core and CdSe shell layers can significantly increase the PCE of CdS/CdSe QDSSC (Yang and Zhong, 2016). Note that all of the solar cells mentioned in literature above were measured under the standard conditions (AM 1.5) and in the presence of a S 2 − /Sx2 − redox couple as electrolyte. Furthermore, another strategy is to increase the PCE of QDSSCs, the doping transition metal (TM) ions, such as Mn2+, Ni2+, Sn4+, into semiconductor nanocrystals (Dai et al., 2014; Debnath et al., 2014; Gopi et al., 2016; Mohammed Panthakkal Abdul et al., 2019), since it is possible to modify the structural, optical, magnetic, electronic and photo-physical properties of QDs which lead to improving the performance of QDSSCs (Bhargava et al., 1994; Klimov et al., 2007). Also, the doping TM ions (usually Mn2+) lead to longer excited state electron life time of the QDs, which results in boosting up the efficiency value of QDSSCs (Halder and Bhattacharyya, 2015; Santra and Kamat, 2012). Our aim in this research is to change and optimize various parameters in QDSSCs with TiO2/CdSe0.2S0.8/ZnS photoanode and Cu2Sbrass as CE in the presence of polysulfide electrolyte to enhance their PCE. So, during this research, this device is considered as a standard cell. Hence, in step one, with deposition of CdSe QDs on the CdSe0.2S0.8 nanocrystals (NCs) surface (as outer QD) by SILAR method, with suitable concentration of Cd2+ and Se2+ sources; and cycle number optimized, a high Jsc of 17.850 mA·cm−2 at AM 1.5 was obtained due to the increased light-absorption and reduced charge recombination, and thereby the PCE augments from 4.327% to 4.918% in comparison with the standard cell. In step two, for the first time, we doped Mn2+ ions in the CdSe QDs deposited on the CdSe0.2S0.8 NCs surface by SILAR method. After optimizing the concentration of Mn2+ ion, the best photovoltaic performance for this device was achieved (5.420%). This improved photovoltaic performance can be considered as a good achievement for the CdSe0.2S0.8 QDSSCs with an improvement of ~25.5%. In this work, to investigate the structure, morphology and optical properties of photoanodes, XRD, Raman, UV–vis, PL and IPCE spectroscopies; and FESEM/EDX, TEM, elemental mapping and AFM techniques have been studied. Also, in order to further investigate the improved efficiency of QDSSCs, electrochemical impedance spectroscopy (EIS) was measured at Voc under standard conditions.

2.2. Preparation of TiO2 films Prior to applying TiO2 paste, FTO conducting glasses were ultrasonically cleaned sequentially in a soap solution, DI water, acetone and ethanol for 15, 20, 15 and 30 min, respectively. Afterwards, the cleaned FTO glasses were soaked in 40 mM aqueous TiCl4 solution at 70 °C for 30 min, then washed with DI water and dried at 100 °C. Finally, TiO2 films containing the transparent and light scattering layers with an approximately thickness of 10.22 µm, as shown in Fig. 4c, were deposited on FTO plates by the doctor-blade method. Then, these films were heated to 120 °C, and sintered at four different temperatures as previously reported (Hossain et al., 2012). In the present work, the active area of the TiO2 films was approximately in range of 0.23–0.28 cm2. 2.3. Preparation of QDs sensitized TiO2 films In this work, all of the TiO2 films were sensitized with the SILAR method except CdSe QDs (CBD) in CdS/CdSe film. For the deposition of CdS QDs, based on previous reports with minor changes (Firoozi and Dehghani, 2016), the TiO2 film was respectively dipped in 0.1 M methanolic solution of Cd(NO3)2 and 0.1 M methanol/ water (1:1 v/v) solution of Na2S for 1 min, followed by rinsing with pure methanol for 1 min and drying with a drier. This procedure was considered one SILAR cycle; it was marked as 1S, which was repeated for five times (5S). For the deposition of CdS/CdSe QDs, CdS QDs were first, as mentioned above CdS QDs, first deposited on the surface of TiO2 film and then CdSe QDs deposited on the CdS(5S)-coated TiO2 film by CBD method. To prepare the chemical bath solution, the aqueous solutions of 80 mM sodium selenosulphate (Na2SeSO3), 80 mM cadmium sulfate (CdSO4) and 120 mM nitriloacetic acid trisodium salt were mixed together. Na2SeSO3 solution was prepared by refluxing 0.08 M selenium powder in an aqueous solution of 0.2 M Na2SO3 at ~80 °C for 7 h under stirring in dark. The CdS(5S)-coated TiO2 film was vertically immersed into the chemical bath solution at 20 °C for 22 h to obtain a suitable amount of CdSe QDs on the CdS-coated film. Then, the film was washed with DI water and dried. For the deposition of n[CdS/CdSe] QDs, similar to what mentioned above, the CdS QDs, first deposited on the TiO2 surface by 1 SILAR cycle and then the CdS(1S)-coated TiO2 film was respectively dipped in 0.3 M methanolic solution of Cd(CH3COO)2 (3 min at room temperature) and 0.3 M aqueous solution of Na2SeSO3 (15 min at 50 °C), followed by rinsing with the corresponding solvent and drying. These procedures were alternately repeated for six times, and the as-prepared electrode was denoted as 6[CdS/CdSe]. For the deposition of CdSe0.2S0.8 QDs, Na2SexS(1-x) solution with Se:S molar ratio of 1:4 was used as an anionic source and 0.5 M methanol solution of cadmium acetate was used as a cationic source. The Na2Se0.2S0.8 anionic solution was prepared by adding an aqueous solution of Na2SeSO3 into aqueous solution of Na2S under vigorous stirring in dark to obtain homogeneous solution with deep red color. For CdSe0.2S0.8 sensitization (hereafter simplified as CdSeS), the TiO2 film was first dipped into the cationic source for 1 min, and then rinsed with pure methanol after 1 min and dried with a dryer. In the next step, the TiO2 film coated with Cd(CH3COO)2 was vertically dipped into the anionic source for 1 min in dark, then rinsed with DI water after 1 min and dried. This two-step procedure was considered one SILAR cycle. In our case, this process was repeated six times (6S) to get the optimum thickness.

2. Experimental section 2.1. Materials The commercially available cadmium nitrate tetra hydrate, cadmium acetate dihydrate, cadmium sulfate, manganese (II) acetate, zinc acetate dihydrate, sodium sulfide nonahydrate, selenium powder, nitriloacetic acid trisodium salt, sulfur powder, sodium sulfite, sodium hydroxide, methanol and ethanol were purchased from Merck and Acros companies and used without further purification. The TiO2 pastes (transparent and scattering), SnO2:F (FTO)-coated (~16 Ω/cm2, 80% 2

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3. Results and discussion

2.3.1. Growth of double-layer QDs of CdSeS/CdSe and CdSeS/CdMnSe For CdSeS/CdSe sensitization, as mentioned in earlier section, CdSeS QDs, first deposited on the surface of TiO2 film and then CdSe QDs as an outer layer were deposited by immersion of the CdSeS(6S)coated TiO2 film into methanol containing Cd(CH3COO)2 (as the Cd2+ cation source) and transparent aqueous Na2SeSO3 solution (as the Se2− anion source), respectively, (with the same concentration of sources which were both 0.3 and 0.5 M in our research) for 1 min at room temperature. After each precursor bath, the photoanode was thoroughly rinsed by immersion in the corresponding solvent to remove the excess ions from the surface for 1 min and then dried; this procedure formed 1 SILAR cycle. In our research, this process was repeated 2, 3 and 4 times (2S, 3S and 4S). In the continuation, we have doped Mn2+ ions in the CdSe layer through the SILAR method. Hence, molar percentages of 5, 10, 15 and 20% of manganese (II) acetate were mixed with suitable concentration of Cd(CH3COO)2 methanol solution. At last, all QD-sensitized TiO2 films were dipped into the Zn (CH3COO)2·2H2O and Na2S 0.1 M aqueous solutions for 1 min in two SILAR cycles (2S) in order to improve the stability and reduce the interfacial charge recombination (Zhao et al., 2015).

3.1. Photovoltaic characterization of QDSSCs Fig. 1(a, b, d) illustrate the photocurrent density–voltage (J-V) curves of QDSSCs under the irradiation of one sun intensity (AM 1.5, 100 mW/cm2), and the main photovoltaic parameters (short circuit current density (Jsc, mA·cm−2), open circuit voltage (Voc, V), fill factor (FF) and PCE (η, %)) have been listed in Tables 1–3. Fig. 1a shows the JV curves of QDSSCs based on CdS(5S), 6[CdS/CdSe], CdS(5S)/CdSe (CBD), and CdSe0.2S0.8(6S) QDs, and the values of photovoltaic parameters for these devices are listed in the Table 1. In comparison, the cell made with the CdSe0.2S0.8-sensitized electrode shows the best performance, with a Voc of 0.55 V, a Jsc of 15.865 mA·cm−2, a FF of 0.496 and a PCE of 4.327%. So, based on the photovoltaic measurements, we demonstrated that the best configuration of the CdS and CdSe QDs in QDSSCs is the form of alloyed them (CdSexS1-x) compared to n[CdS/ CdSe] and CdS/CdSe forms. Hence, in this study we considered the ternary alloy QDs of CdSexS(1-x), with Se:S molar ratio of 1:4, as sensitizer photoanode in the standard cell. For ameliorate the photovoltaic performance of this cell, in step one, we placed CdSe QDs on the TiO2/ CdSeS film surface via SILAR method. The J-V curves of TiO2/CdSeS/ CdSe/ZnS QDSSCs with different conditions for deposition CdSe QDs are shown in Fig. 1b and the corresponding photovoltaic parameters are summarized in Table 2. They show, for device based on CdSeS/CdSe (0.5 M, 2S) sensitizer, Jsc increases from 15.577 to 16.685 mA·cm−2 compared to the CdSeS/CdSe(0.3 M, 2S) device, and consequently the PCE improves (from 4.327% to 4.538%). As a result, we deposited CdSe QDs with 0.5 M concentration of Cd2+ and Se2− sources on the CdSeS QDs surface by various SILAR cycles. According to Fig. 1b and Table 2, it is clear that increasing the thickness of CdSe layer (owing to increased number of SILAR cycles) leading to a higher Jsc and a lower Voc, so the performance of a CdSeS/CdSe(0.5 M) QDSSC is very much dependent on the number SILAR cycles to deposit CdSe QDs. As listed in Table 2, when the CdSe QDs with 3 SILAR cycles loading on TiO2/ CdSeS surface, although the FF remains almost unchanged (0.492), the Voc and Jsc values were improved and reached to 0.56 V and 17.850 mA·cm−2, respectively. As a result, the PCE value of QDSSCs ultimately increased with the improved Voc and Jsc and reached to 4.918%. In continuation, when 4 SILAR cycles of CdSe QDs are deposited on the surface of CdSeS QDs, although the Jsc increase to 20.302 mA·cm−2, the values of Voc, FF and thereby the PCE were all decreased to 0.49 V, 0.393 and 3.909%, respectively. This decrease of PCE is due to the increased thickness of CdSe layer in the CdSeS/CdSe(0.5 M, 4S) cell, which would increase the number of excited electrons and thereby the recombination of electron-hole pairs within the QDs and at the electrode/electrolyte interface (Hou et al., 2018). Eventually, increasing the number of SILAR cycles from 3 to 4 leads to an increase in the Jsc and a decrease in the Voc and PCE. Among all the QDSSCs in Table 2, the CdSeS/CdSe(0.5 M, 3S) sensitizer solar cell show the best photovoltaic performance with a 13% increase of PCE compared to bare CdSeS device. This enhancement of PCE in CdSeS/CdSe(0.5 M, 3S) cells could be attributed to the wider light absorption region and reduced recombination of excited electrons with holes in the polysulfide electrolyte (Fig. 2, red1 arrow) compared to the standard cell, which is due to the formation of suitable cascade energy band structure for electron delivery and hole-recovery by deposition of the CdSe outer QD (Chen et al., 2014). To further confirm the enhanced short circuit current of the CdSeS/ CdSe QDSSCs, incident photon-to-current conversion efficiency (IPCE, or external quantum efficiency, EQE) measurements have been

2.4. Preparation of QDSSCs In all work, the cells were prepared by assembling the sensitized photoanode, Cu2S on brass foil as CE and a scotch spacer (thickness 50 µm), and polysulfide electrolyte with PH ~ 12.8 (containing 1 M S, 1 M Na2S·9H2O and 0.1 M NaOH in DI water which was stirred in dark for 6 h). The Cu2S-brass CEs were prepared by dipping clean brass foil in 37% HCl at 80–90 °C for 45 min to expose more copper from the copper-zinc matrix. Then, one drop of a polysulfide aqueous solution (containing 1 M S and 1 M Na2S) was added onto a part of it leading to the sudden formation of a black Cu2S layer on the brass foil.

2.5. Characterization All analyses were achieved at room temperature under environment conditions. The surface morphology and chemical composition of the QD-sensitized TiO2 films were characterized with a field emission scanning electron microscope (FESEM, MIRA3 TESCAN and Philips XL30FESEM) equipped with an energy-dispersive X-rey (EDX) spectrometer. Also, transmission electron microscopy (TEM, Philips CM30) images were used to more investigate the deposition of CdMnSe QDs on the CdSeS surface. The crystalline structure of films was recorded by Xray diffraction (XRD, Philips-X'pertpro, X-ray diffractometer via NiFiltered Cu kα radiation) and Raman (Takram P50CR10, laser Nd-YAG) spectroscopies. The surface roughness of the photoanodes was investigated using an atomic force microscope (AFM), NT-MDT, building 167, zeleno grad, by tapping mode. UV–visible diffuse-reflectance spectroscopy (DRS) of films was recorded using Uv-1800 spectrophotometry. The photoluminescence spectra (PL) of the photoanodes were measured by a Hitachi 850 fluorescence spectrophotometer. The pH level of solutions was determined by using a pH meter (metrohm). The incident photo-to-current efficiency (IPCE) spectra were measured in the wavelength range of 375–900 nm using a 300 W xenon lamp. The photocurrent density–voltage (J-V) characteristic curves of the QDSSCs were measured by a Keithley model 2400 digital source meter (Keithley, USA) under the illumination of AM 1.5 with a power density of 100 mW/cm2. The electrochemical impedance spectra (EIS) of the QDSSCs were obtained on the Voc under a standard condition (AM 1.5) with potantiostat/galvanostat (PGSTAT 100, Autolab, Eco-Chemie), at an AC amplitude of 5 mV within the frequency range from 0.01 Hz to 500 kHz.

1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.


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Fig. 1. J–V characteristics of QDSSCs with (a) CdS(5S), 6[CdS/CdSe], CdS(5S)/CdSe (CBD) and CdSe0.2S0.8(6S) QDs; and (b) CdSeS/CdSe/ZnS QDSSCs with two various concentrations of Cd2+ and Se2− sources in CdSe QDs, and different CdSe SILAR cycles under 1-sun illumination (100 mW/cm2). (c) The IPCE spectra of CdSeS and CdSeS/CdSe (0.5 M, 3S) QDSSCs. (d) J-V characteristics of CdSeS/CdMnSe/ZnS QDSSCs with different molar percentages of Mn2+ ions doped in CdSe QDs under 1-sun illumination (100 mW/cm2).

Table 1 Photovoltaic performance of QDSSCs made with different QD-sensitizer electrodes under one full sun condition (100 mW/cm2). Photoanode

Voc (V)

Jsc (mA·cm−2)


PCE (η , %)

CdS(5S) 6[CdS/CdSe] CdS(5S)/CdSe(CBD) CdSe0.2S0.8(6S)

0.52 0.51 0.52 0.55

5.668 13.713 16.856 15.865

0.482 0.425 0.458 0.496

1.420 2.972 4.014 4.327

Table 2 Photovoltaic performance of QDSSCs based on CdSeS(6S)/CdSe/ZnS(2S) QDs with two various concentrations of Cd2+ and Se2− sources in CdSe QDs, and different number of CdSe SILAR cycles under one full sun condition (100 mW/ cm2). Photoanode CdSeS/CdSe(0.3 M, CdSeS/CdSe(0.5 M, CdSeS/CdSe(0.5 M, CdSeS/CdSe(0.5 M,

2S) 2S) 3S) 4S)

Voc (V)

Jsc (mA·cm−2)


PCE (η , %)

0.58 0.58 0.56 0.49

15.577 16.685 17.850 20.302

0.479 0.469 0.492 0.393

4.327 4.538 4.918 3.909

Fig. 2. Ideal stepwise structure of band edges and midgap states of Mn2+ for the electrons and holes transfer mechanism in TiO2/CdSeS/CdMnSe QDSSCs.

3S) QDs is somewhat broader and higher than that of the IPCE curve of bare CdSeS based QDSSC. This disparity reveals that light harvest capacity of CdSeS/CdSe(0.5 M, 3S) cell is much higher than that of the standard cell. Based on these spectra, the calculated integrated Jsc value for the CdSeS and CdSeS/CdSe(0.5 M, 3S) sensitized solar cells are 15.44 and 17.52 mA·cm−2, respectively, which are in good agreement with the values obtained from the J-V curves (Tables 1 and 2). Also, from this the enhanced Jsc of CdSeS/CdSe(0.5 M, 3S) cell after overcoating with CdSe QDs, it is found that the presence of CdSe outer QD in this photoanode configuration can help to enhance the light harvest capacity of device. After optimizing the thickness of the CdSe layer, we studied the influence of doped manganese ion in this layer of TiO2/CdSeS(6S)/CdSe (0.5 M, 3S) photoanode on the QDSSCs performance. The J-V curves of the QDSSCs with different molar percentages of Mn2+ ion (5% (0.025 M), 10% (0.05 M), 15% (0.075 M) and 20% (0.1 M)) are plotted in Fig. 1d and detailed parameters are summarized in Table 3. According to Fig. 1d and Table 3, the QDSSC based on the CdSeS/

Table 3 Photovoltaic performance of QDSSCs based on CdSeS(6S)/CdMnSe(0.5 M, 3S)/ ZnS(2S) QDs with various molar percentages of Mn2+ ion doped in CdSe outer layer under one full sun condition (100 mW/cm2). Photoanode

Voc (V)

Jsc (mA·cm−2)


PCE (η , %)

5% CdMnSe 10% CdMnSe 15% CdMnSe 20% CdMnSe

0.73 0.70 0.58 0.59

15.590 16.834 17.047 17.934

0.474 0.460 0.469 0.382

5.394 5.420 4.637 4.041

conducted, and the results are presented in Fig. 1c. This figure shows the IPCE spectra of QDSSCs with CdSeS and CdSeS/CdSe(0.5 M, 3S) photoanodes in the range of 375–900 nm. From these spectra, it could be found that the IPCE curve of the QDSSC based on CdSeS/CdSe(0.5 M, 4

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QDs on the surface of CdSeS that leads to a little increase in particle size and creating more pores in the surface which can be favorable for the easy penetration of ions during dope process. These exchanges have shown the CdSe QDs deposited on the surface of the CdSeS film. It is obvious in Fig. 3(c, c1), more of the pores in Fig. 3(b, b1) are filled due to the successful doping of Mn2+ ions in CdSe layer (CdMnSe). This exchange is a sign that a larger amount of Mn2+ ion was incorporated into the CdSe QDs. Furthermore, the surface coverage with ZnS passivation layer leads to further reduction of the pores in TiO2/CdSeS/ CdMnSe film as showed in Fig. 3(d, d1). Hence, filling the pores with nanoparticles is expected to simplify the penetration of electrolyte in TiO2/CdSeS/CdMnSe/ZnS photoanode (Gopi et al., 2015), the result of which is an improvement in QDSSC efficiency. This result is in good agreement with the J-V data in Section 3.1. Also to more investigate of the deposition of CdMnSe QDs on the CdSeS surface, the TEM images of the CdSeS and CdSeS/CdMnSe QD-sensitized TiO2 films in the different scales are shown in Fig. 4(a, b) (a1, b1). It can be clearly seen that the CdMnSe QDs as outer QDs are deposited on the surface of TiO2/CdSeS.

5%CdMnSe photoanode exhibited a better photovoltaic performance compared to the cell without dopant with the values of 0.73 V, 15.590 mA·cm−2, 0.474, and 5.394% for Voc, Jsc, FF, and PCE, respectively. Then, with the increase in Mn2+ doping molar percentages from 5% to 10%, while the FF slightly decreased, a significant improvement in the values of Voc, Jsc and thereby PCE happens, and a superior power conversion efficiency of 5.420% is obtained with the photovoltaic parameters of Voc = 0.70 V, Jsc = 16.834 mA·cm−2 and FF = 0.460. In this concentration of Mn2+ ion (10% (0.05 M)), we observed an overall conversion of 25% in Voc after Mn2+ doping, which is the main cause of the enhancement of the efficiency from 4.918% (Mn-free cell) to 5.420% (Mn-doped cell), an improvement of 10%. In contrast, when the molar percentage of Mn2+ ion increases from 10% to 15%, while Jsc slightly improved from 16.834 to 17.047 mA·cm−2 and FF remains unchanged (~0.46), the Voc and PCE values significantly decrease from 0.70 to 0.58 V and from 5.420 to 4.637%, respectively. Also, a further addition of doping amount to 20%, leads to a further reduction in the photovoltaic performance of device, which is due to the lattice distortion (Veerathangam et al., 2017). In result, the lowest PCE value of QDSSC based on CdSeS/ 20%CdMnSe photoanode with Voc of 0.59 V, Jsc of 17.934 mA·cm−2, FF of 0.382, and PCE of 4.042% are obtained. Generally, based on these observations, with the increase in Mn2+ doping amount (from 5% (0.025 M) to 20% (0.1 M)), PCE first increases and then decreases, and eventually the highest PCE value of 5.420% was obtained when the Mn2+ doping amount was 10% or 0.05 M. Therefore, the molar percentage of Mn2+ ion in this photoanode configuration is optimized to 10% and this amount is fixed for the following characterizations.

3.2.2. Study of the presence of Mn2+ ions into CdSe QDs In the present research, the XRD, Raman and EDX spectroscopies, and elemental mapping images of the TiO2/CdSeS/CdMnSe film is used to more investigate of the presence of Mn2+ ions with 10% doping concentration in CdSe QDs. Fig. 5a depicts the XRD patterns of the TiO2/CdSeS and TiO2/CdSeS/CdMnSe films. The XRD patterns of these films show the diffraction peaks at 2θ values of ~25, 27, 38, 48, 54 and 56, which correspond to the anatase (JCPDS card no. 21-1272) and rutile (JCPDS card no. 21-1276) TiO2 phases. In the XRD patterns, the diffraction peaks of the CdSeS QDs are not present, which could be due to the low amount these QDs loaded by 6 SILAR cycles (6S) on the TiO2 films (Hou et al., 2018). The formation of CdSe in TiO2/CdSeS/CdMnSe film was confirmed by the appearance of peaks at 2θ of ~27° and 42° (JCPDS card no. 77-2307). In here, the characteristic diffraction peak of MnSe is not present, which is due to the low amount of Mn2+ doped in the CdSe QDs. Room temperature Raman spectra of the un-doped and Mn-doped CdSe QDs on the surface of TiO2/CdSeS films are shown in Fig. 5b. It shows the peaks at positions of ~144, 200, 290, 400 and 500 cm−1, which these peaks corresponding to the CdSeS and CdSe QDs, and anatase TiO2 phase (Dzhagan et al., 2013; Lagopati et al., 2014). It is obvious that after the doping Mn2+ ions, the two peaks in the Raman spectrum of TiO2/CdSeS/CdMnSe film appear at ~360 and 459 cm−1 positions that corresponding to the MnSe formation due to

3.2. Photoanodes characterization In this section, we investigate structural characterization of the TiO2/CdSeS(6S), TiO2/CdSeS(6S)/CdSe(0.5 M, 3S), TiO2/CdSeS(6S)/ 10%CdMnSe(0.5 M, 3S) and TiO2/CdSeS(6S)/10% CdMnSe(0.5 M, 3S)/ ZnS(2S) photoanodes on the FTO substrate. 3.2.1. Study of the structure and morphology In order to study the surface morphology and structure of the photoanodes, the FESEM and TEM were used and the results were displayed in Fig. 3(a–d) (a1–d1) and 4(a, b) (a1, b1). It can be seen in Fig. 3(a, a1) that the surface of TiO2/CdSeS film is composed of nanoparticles with a close-packed morphology and relatively homogeneous dispersion. Fig. 3(b, b1) correspond to the deposition of CdSe

Fig. 3. FESEM images of (a, a1) CdSeS, (b, b1) CdSeS/CdSe, (c, c1) CdSeS/CdMnSe and (d, d1) CdSeS/CdMnSe/ZnS QDs on the surface of TiO2. 5

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Fig. 4. TEM images of (a, a1) CdSeS and (b, b1) CdSeS/CdMnSe QDs deposited on TiO2 nanoparticles. (c, c1) Cross-sectional SEM images of the TiO2 film containing the transparent and scattering layers.

Fig. 5. (a) XRD patterns of the CdSeS and CdSeS/CdMnSe QDs sensitized TiO2 films. (b) Raman spectra of undoped and Mn-doped CdSe QDs on the surface of TiO2/ CdSeS. (c) EDX spectroscopy of TiO2/CdSeS/CdMnSe/ZnS photoanode. (d) FESEM and (d1) elemental mapping images of the TiO2/CdSeS/CdMnSe/ZnS film. 6

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3.2.4. Study of the optical properties In order to study the optical properties of CdSeS, CdSeS/CdSe and CdSeS/CdMnSe QD-sensitized photoanodes by DRS and PL spectra, only the transparent TiO2 layer was used in electrodes. According to Fig. 7a, with deposition of the CdSe QDs on the CdSeS surface which led to the deep color of TiO2/CdSeS photoanode (inset of Fig. 7a), the absorbance increases compared to bare CdSeS photoanode. In addition, when 10% of Mn2+ dopant was incorporated onto the surface of CdSe QDs, the absorbance slightly further increases compared to Mn-free film. As a result, the light absorption intensity in bare CdSeS QD-sensitized photoanode can be improved with employing CdMnSe outer QD, and the result could increase the Jsc value and thereby the enhancement of the PCE of the QDSSC. These results are consistent with the J-V data. According to Fig. 7b, the PL emission peak of CdSeS/CdSe QDs is markedly quenched after CdSe QDs deposition, which indicates the recombination of electrons and holes is significantly reduced (Eskandarloo et al., 2014). But after the introduction of Mn2+ ions into CdSe QDs; the PL emission intensity of CdSeS/CdSe QDs slightly increased. In this case, the possible cause is due to the fact that the Mn2+ doped into CdSe QDs can cover an emission window similar to that of the current workhorse of intrinsic QDs emitters, which leads to producing more excitations in the CdSe QDs and thereby the increase of the PL of QDs (Pradhan and Xiaogang, 2007). The increase of PL enhances the emission quantum yield (QY) of QDs, which leads to producing more exctions (electrons and holes) and thereby the higher recombination of electrons and holes to emit large number photons. However, in the QDSSCs, the electrons are often transferred to the metal oxide electrodes instead of the recombination with holes, which this procedure is the main cause of the enhancement of the charge density in these cells (Tian et al., 2014; Asgari Fard and Dehghani, 2019). So, it may be anticipated that dopant Mn2+ can improve the electron transfusion from QDs to TiO2 CB, and leads to a high charge density in the TiO2 CB, as shown in Fig. 2. Finally, according to Fig. 7b, it is evident that the PL emission peak of bare CdSeS QDs is sharply quenched despite the increase in absorbance after loading with CdMnSe outer QD, which indicates the recombination of electron-hole pairs is significantly reduced. Hence, it may be anticipated that the device fabricated using CdMnSe outer QD will provide the higher efficiency compared to bare CdSeS device. This result is consistent with the results of the J-V data.

Fig. 6. 3D AFM images of (a) CdSeS, (b) CdSeS/CdSe, (c) CdSeS/CdMnSe and (d) CdSeS/CdMnSe/ZnS films on the surface of TiO2 (containing the transparent and scattering layers).

the successful doping of Mn2+ ions in CdSe QDs (Popović and Milutinović, 2006). In addition, the presence of Mn peaks (Lα, Kα and Kβ) at energy levels of ~0.650, 5.89 and 6.539 KeV in the EDX spectrum of the TiO2/CdSeS/CdMnSe/ZnS film (Fig. 5c), and also the existence of Mn elements in the elemental mapping image of this film (Fig. 5d1-Mn) are another confirmation for incorporation of Mn2+ ions into CdSe QDs.

3.2.3. Study of the surface roughness Herein, the surface roughness of the photoanodes was investigated by atomic force microscopy (AFM) technique, and the corresponding 3dimensional (3D) images with different morphologies of surface are shown in Fig. 6a–d. According to these Figures, after the deposition of CdSe layer on the surface of the TiO2/CdSeS film, the root mean square (RMS) surface roughness value decreases from 272 nm (Fig. 6a) to 143 nm (Fig. 6b). Furthermore, with the doping 10% of Mn2+ ion into the CdSe layer, the RMS value further decreases (77 nm, Fig. 6c). Finally, the lowest RMS value was obtained for the TiO2/CdSeS/CdMnSe/ ZnS film (48 nm, Fig. 6d), which is due to the surface coverage with a thin passivation layer of ZnS (the smoothest surface). As the result of this investigation, the deposition of CdMnSe layer on the surface of the TiO2/CdSeS photoanode leads to improving surface roughness and morphology, which can be favorable for the easy penetration of electrolyte in the photoanode and results in increasing the efficiency of the QDSSC (Gopi et al., 2015). Thus, these results agree well with the FESEM and J-V curves data.

3.3. EIS characterization of QDSSCs The electrochemical impedance spectroscopy (EIS) was measured to further investigate the influence of CdSe and CdMnSe QD layers deposited on the surface of TiO2/CdSeS electrode for the observed enhanced PCE under 1 sun illumination. Fig. 8a shows the Nyquist curves of QDSSCs based on TiO2/CdSeS/ZnS, TiO2/CdSeS/CdSe/ZnS and TiO2/CdSeS/CdMnSe/ZnS photoanodes in the frequency range of

Fig. 7. (a) DRS and (b) PL spectra of CdSeS, CdSeS/CdSe, CdSeS/CdMnSe QDs-sensitized TiO2 electrodes consisting only a transparent thin layer. 7

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(23.61 Ω·cm2) is larger than the bare CdSeS based cell (8.49 Ω·cm2). Thus, the charge recombination resistance at the TiO2/electrolyte interface enhanced after over-coating with CdSe QDs. Also, this table shows the electron lifetime (τe ) values of QDSSCs in the TiO2 film, which is estimated from equation τe = 1/2πfmax . Where fmax is the maximum frequency of the main peak in Bode plots (Fig. 8b), which has been related to the second semicircle in the Nyquist curves (Fig. 8a) (Sarker et al., 2014; Wang et al., 2005). As seen in Table 4, the electron lifetime value rises from 0.104 s in CdSeS QDSSC to 0.173 s in the CdSeS/CdSe QDSSC. This enhancement in τe value can be attributed to the decreased charge recombination in CdSeS/CdSe photoanode that is due to the best electron transport path from the CB of CdSe (in highenergy level) to CdSeS CB (in low-energy level), and finally at the CB of TiO2 nanoparticles, the cascade band structure, as shown in Fig. 2. The effective rate constant for recombination (keff ) is determined by the electron lifetime from equation keff = 1/ τe (Wang et al., 2005). The results show that the value of keff of the CdSeS/CdSe (5.78 s−1) is lower than the CdSeS (9.61 s−1). This ~66% decrease of keff confirms that the presence of CdSe QDs effectively reduced recombination of electrons with holes in the electrolyte. As a result, parallel increase of RCT and τe values in CdSeS/CdSe QDSSC can increase the Jsc and then the efficiency compared to CdSeS-based cell, which these results agree well with the IPCE and PL spectra, and J-V data of CdSeS and CdSeS/CdSe QDSSCs. Also, EIS analysis was applied to examine the ability of CdMnSe layer as outer QD in CdSeS QDSSC to raise the efficiency of the solar cell. According to Table 4, with applying CdMnSe outer QD in CdSeS QDSSCs, the RCT value increases from 8.49 to18.80 Ω·cm2. The electron density (ns ) which represents the electrons accumulation in TiO2 CB can be obtained by a parameter ʻCon ’ (Phadke et al., 2011; Afrooz and Dehghani, 2016), in the following way: Fig. 8. (a) Nyquist curves and (b) Bode plots of QDSSCs based on CdSeS, CdSeS/CdSe and CdSeS/CdMnSe QDs under 1-sun illumination (100 mW/cm2). The inset of plot (a) shows equivalent circuit diagram used for fitting the EIS spectrum.

Con = RCT Lkeff = kB T / q2Ans


Here, L , kB , T , q and A represent the film thickness, Boltzman constant, absolute temperature, charge of a proton and the electrode area, respectively. Based on the calculated results, the ns value rises from 1.92E +18 cm3 in bare CdSeS to 2.41E+18 cm3 in CdSeS/CdMnSe cell, which is caused by better accumulation of electrons in TiO2 CB. The high ns value and increment of the RCT value in CdSeS/CdMnSe cell compared to CdSeS cell indicate that the electron recombination process is suppressed by applying CdMnSe outer QD that may be the major reason of higher Voc and the improvement of the performance in this solar cell (Hou et al., 2016; Santra and Chen, 2014). These results are in agreement with the J-V data and the PL spectra obtained for these cells in Sections 3.1 and 3.2. Also, the enhanced τe and decreased keff in CdSeS/ CdMnSe cell compared to CdSeS cell support the reduction of backreaction for the injected electrons with electrolyte.

100 mHz to 500 kHz. All the Nyquist curves were fitted by the equivalent circuit (the inset in Fig. 8a) using Z-view software (v2.9c, Scribner Associates Inc.), and the fit data are listed in Table 4. The EIS spectra of three QDSSCs in Fig. 8a show two semicircle curves with increasing frequency. The first semicircle, corresponding to the high frequency (left) region, is related to the electron injection at the CE/ electrolyte interface and transport in the electrolyte (R1 = RCE ) (Tian et al., 2014; Koide et al., 2006). The second semicircle in the intermediate frequency (right) region corresponds to the charge transfer at the electrode/electrolyte interface and transport in the TiO2 film (R2 = RCT ) (Firoozi and Dehghani, 2016; Zhao et al., 2015). The RCT is considered as the charge recombination resistance in the QDSSC (Rrec ) (Park et al., 2010; Park et al., 2011). In addition, the starting point of the first semicircle (in the high frequency range) is ohmic series resistance (RS ), which is related to the FTO sheet resistance, TiO2/FTO contact resistance (Bisquert et al., 2009) and all resistance outside of the device (Zhao et al., 2015). According to Table 4, the RCE value of CdSeS (1.22 Ω·cm2) is lower than the similar values of CdSeS/CdSe (2.72 Ω·cm2) and CdSeS/CdMnSe (3.06 Ω·cm2). The results suggest that the electron injection at the CE/ electrolyte interface was more efficient in the presence of the CdSe layer. The RCT value of a QDSSC based on CdSeS/CdSe QDs

4. Conclusions The results show that after the deposition of CdSe QDs by SILAR method (with suitable and same concentration of Cd2+ and Se2− sources (0.5 M); and optimized cycle number (3S)) onto the CdSe0.2S0.8 QDs-sensitized TiO2 film, the cascade energy band structure of TiO2/ CdSe0.2S0.8/CdSe is formed, and its PCE is increased by 13%. Based on IPCE, DRS, PL, EIS and J-V data results, this enhancement is related to the increasing light absorption and the decreasing recombination of electrons in QDSSC that led to the increase of Jsc and PCE in this cell. To

Table 4 Electrochemical impedance results of CdSeS, CdSeS/CdSe and CdSeS/CdMnSe sensitized QDSSCs under one full sun condition (100 mW/cm2). Photoanode

RS (Ω·cm2)

RCE (Ω·cm2)

RCT (Ω·cm2)

τ (s)

k eff (s−1)

Con (Ω·cm/s)

ns (cm3)E+18

CdSeS CdSeS/CdSe CdSeS/CdMnSe

2.48 2.32 1.99

1.22 2.72 3.06

8.49 23.61 18.80

0.104 0.173 0.288

9.61 5.78 3.47

0.36 0.49 0.29

1.92 1.15 2.41


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further increase the performance of this device, we doped Mn2+ into the CdSe QDs. After optimizing the concentration of Mn2+ ions, the best efficiency for the CdSe0.2S0.8/10%CdMnSe device with an improvement of ~25.5% in comparison with bare CdSe0.2S0.8 alloy QDs was obtained (5.420%), which is the highest reported value for CdSexS1-x/CdSe QDs. Based on our experimental results, source of increasing efficiency in the CdSe0.2S0.8/10%CdMnSe QDSSC compared to bare CdSe0.2S0.8 QDSSC was originated from increasing Voc that is due to the increasing collection of electrons in the TiO2 CB and the decreasing recombination of electrons, as observed by PL and EIS spectroscopies, and J-V data. The reported results demonstrated that employing CdMnSe layer as an outer QD in CdSe0.2S0.8 alloyed QDSSCs is a simple and effective strategy to enhance light absorption, charge recombination control and thereby the improvement of the PCE of solar cells.

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